Implementation of a null test for freeform optics using a high-definition spatial light modulator.

Area of Science:

• Optical metrology
• Interferometry
• Freeform optics

Background:

• Traditional interferometric null tests often rely on static computer-generated holograms (CGHs).
• CGHs can be costly and time-consuming to fabricate for unique optical components.
• Reconfigurable optical elements offer a potential solution for flexible metrology.

Purpose of the Study:

• To implement and validate an interferometric null test system utilizing a spatial light modulator (SLM).
• To demonstrate the SLM's capability as a reconfigurable alternative to CGHs in optical testing.
• To assess the system's accuracy in measuring complex optical surfaces, including freeforms.

Main Methods:

• Development of an interferometric null test system incorporating a high-definition spatial light modulator (SLM).
• Implementation of a detailed alignment procedure, including novel SLM-based fiducial projection.
• Validation of the alignment protocol by measuring an off-axis conic and comparing results with conventional interferometry.
• Testing of a 65 mm clear aperture concave freeform optic with significant sag departure.

Main Results:

• The SLM-based system achieved cross-validation with conventional interferometry within 30 nm root-mean-square (RMS) for surface figure measurement.
• Accurate null testing of a concave freeform optic was demonstrated.
• The measured surface figure of the freeform optic was within 40 nm RMS of a commercial metrology instrument's results.

Conclusions:

• A spatial light modulator (SLM) provides a viable and reconfigurable alternative to computer-generated holograms for interferometric null testing.
• The developed SLM-based system offers high precision for measuring complex optical surfaces, including freeforms.
• This approach enhances flexibility and potentially reduces costs in optical metrology applications.